linux-kernel - Re: [PATCH x86/mm 6/6] x86-64 ia32 ptrace get/putreg32 current task

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <alpine.LFD.0.9999.0711291057320.8458@woody.linux-foundation.org>
Date:	Thu, 29 Nov 2007 11:08:41 -0800 (PST)
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	"H. Peter Anvin" <hpa@...or.com>
cc:	Chuck Ebbert <cebbert@...hat.com>,
	Roland McGrath <roland@...hat.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	linux-kernel@...r.kernel.org, Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...e.hu>
Subject: Re: [PATCH x86/mm 6/6] x86-64 ia32 ptrace get/putreg32 current
 task

On Thu, 29 Nov 2007, H. Peter Anvin wrote:
> Linus Torvalds wrote:
> > 
> > > It is advantageous for user space to use the register the kernel typically
> > > won't, in order to speed up system call entry/exit.
> > 
> > but I'm not seeing the reason for that one. Care to comment more? (Yes,
> > there is often a latency from segment reload to use, but the reload latency
> > for system call exit *should* be entirely covered by the cost of doing the
> > system call return itself, no?)
> 
> I do seem to recall that some processor implementations can load a NULL
> segment faster than a non-NULL segment.  This was significant enough that we
> wanted to use %fs in x86-64 userspace, as opposed to the original ABI which
> used %gs both in userspace and in the kernel.

Ahh, I think you may be right for some CPUs. The zero selector is indeed 
potentially faster to load, since it doesn't have to even bother looking 
at the GDT/LDT.

That said, I doubt it's very noticeable. I just ran tests on both an old 
P4 and on a more modern Core 2 machine, and for both of those the 
performance was identical between loading a NUL selector and loading it 
with a non-zero one.

But I could well imagine that it matters a few cycles on other CPU's. But 
from my testing, it definitely isn't noticeable, and I think the 
maintenance advantage of using the same segment setup would more than make 
up for the fact that maybe some odd CPU can see a difference.

			Linus
-
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@...r.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/